The invention generally relates to implantable cardioverter defibrillators (ICD""s)and in particular to techniques for determining and setting defibrillation thresholds for use with ICD""s.
An ICD is an implantable medical device capable of detecting the onset of ventricular fibrillation, or related arrhythmias, and for administering a defibrillation electrical pulse directly to the heart tissue to terminate the fibrillation. Preferably, the ICD is configured to discharge the minimum amount of electrical energy necessary to reliably defibrillate the heart. The amount of energy discharged in a single defibrillation pulse is the xe2x80x9cdefibrillation dosagexe2x80x9d. By keeping the defibrillation dosage to a minimum: possible injury to the heart tissue caused by the electrical pulse is avoided; there is less discomfort to the patient after the patient has been revived; and the longevity of the power supply of the ICD is enhanced.
The defibrillation dosage must be sufficient to generate an xe2x80x9cactual defibrillation voltagexe2x80x9d in the heart of the patient sufficient to overcome a threshold xe2x80x9ctissue defibrillation voltagexe2x80x9d of the patient. The tissue defibrillation threshold is the minimum electrical voltage gradient that must be induced within the muscle tissue of the heart to reliably defibrillate the tissue. The tissue defibrillation threshold is typically about five volts per centimeter but can vary depending upon the characteristics of the heart, particularly the extent to which the tissue of the heart has been damaged by previous incidents of fibrillation or other arrhythmias.
The actual defibrillation voltage induced in the heart from a defibrillation pulse depends upon: the strength of the pulse generated by the ICD; the configuration of the ICD and its components, particularly the size, type and location of the leads of the ICD; and the physical characteristics of the heart and thorax of the patient, particularly the size and shape of the heart, the size and shape of the thorax, and the amount of fat present in the thorax. Accordingly, a particular defibrillation dosage administered by an ICD can result in significantly different actual defibrillation voltages within the heart tissue depending upon characteristics of the patient and upon the configuration of the ICD implanted in the patient.
Additional information regarding defibrillation dosages and thresholds may be found in chapters 4 and 5 of xe2x80x9cImplantable Cardioverter Defibrillator Therapyxe2x80x94The Engineeringxe2x80x94Clinical Interface,xe2x80x9d edited by Mark W. Kroll and Michael H. Lehmann, Kluwer Academic Publishers (1996).
As noted, it is desirable to minimize the defibrillation dosage administered by the ICD. Hence, it is desirable to first determine an optimal implantation configuration for the ICD and its components which achieves the highest actual defibrillation voltage within the heart tissue using the lowest defibrillation dosage. The optimal configuration also minimizes overall costs and maximizes ICD longevity. The optimal configuration is one that not only specifies the components to be used, including the make and model of the ICD, the leads etc., but also specifies location in which each component is to implanted. The optimal configuration is, of course, preferably determined prior to implantation of the ICD and its components into the patient.
Heretofore, unfortunately, there has been no expedient, reliable and inexpensive noninvasive technique for determining the optimal ICD implantation configuration for a particular patient. As a result, the physician may erroneously select an implantation configuration which requires that the ICD discharge unnecessarily large amounts of electrical energy within each defibrillation pulse, thereby possibly damaging heart tissue, reducing the longevity of the ICD and increasing the amount of pain the patient experiences after being revived. A non-optimal implantation configuration may also result in greater costs and overall discomfort to the patient, particularly if a large and expensive ICD is implanted, when a smaller and less expensive one would be sufficient if implanted using an optimal configuration of leads, auxiliary wires, and the like.
Thus, it would be highly desirable to provide an improved technique for determining an optimal ICD implantation configurationxe2x80x94specifying the components to be used and the location in which each component is to implantedxe2x80x94based on the characteristics of a particular patient. It is to this end that aspects of the invention directed.
Once an ICD and its peripheral components have been selected and implanted, the defibrillation dosage of the ICD must be set to an amount sufficient to reliably defibrillate the heart, i.e. the defibrillation dosage must be set to an amount sufficient to achieve an actual defibrillation voltage gradient within the heart tissue exceeding the tissue defibrillation threshold. Herein, the minimum ICD defibrillation dosage sufficient to reliably defibrillate the heart is referred to as the xe2x80x9cdosage defibrillation thresholdxe2x80x9d or DFT. Typically, the DFT is a programmable parameter of the ICD. In use, if ventricular fibrillation is detected, the ICD initially attempts to defibrillate the heart using a defibrillation dosage set to the DFT. If ventricular fibrillation is not terminated using pulses at the DFT, the ICD generates one or more additional pulses of higher dosage, typically using the maximum energy available.
As noted above, it is desirable to employ the lowest defibrillation dosage possible to reliably defibrillate the heart. Hence, the optimal setting for programming the DFT of the ICD is the lowest DFT value sufficient to achieve reliable defibrillation, which, as also noted, can depend greatly on the characteristics of the patient, such as the shape and size of the heart and thorax, and on the configuration of the ICD and its components. Thus, the optimal value may vary greatly from patient to patient and, for a particular patient, may vary greatly depending upon the ICD implantation configuration. Unfortunately, heretofore, there has been no reliable, inexpensive and expedient technique for accurately determining the optimal value for programming the DFT of an ICD based upon the characteristics of the patient and upon the ICD implantation configuration.
Conventionally, to program the DFT of an ICD, ventricular fibrillation is induced within the patient, then a series of defibrillation pulses of differing dosages are administered to the heart in an attempt to terminate the fibrillation. The DFT is then programmed to the lowest dosage level that terminated fibrillation, plus some safety margin. In one technique, fibrillation is repeatedly induced and pulses are output with decreasing dosage levels until the dosage level is insufficient to terminate the fibrillation. In another technique, a binary search pattern is employed whereby fibrillation is repeatedly induced, and then defibrillation pulses alternating between high and low output dosages are administered with a voltage differential between the high and low pulses incrementally reduced until an approximate DFT is identified.
Although these techniques have been found to be effective, considerable room for improvement remains. First, once fibrillation is induced, there is a risk that the physician will not be able to subsequently terminate the fibrillation, resulting in loss of life to the patient. Second, the sequence of defibrillation pulses may further injure the cardiac tissue of the patient and certainly can be painful to the patient. Moreover, the overall process typically takes several hours resulting in significant costs. Typically, the ICD itself is employed to administer the electrical defibrillation shocks and, if a relatively large number of shocks are required to determine the DFT, the power supply of the ICD can be depleted considerably during the determination process. If relatively few test defibrillation pulses are used, then the technique can only approximate the correct DFT thereby requiring the physician to set the DFT of the ICD using a high safety margin which, in turn, can ultimately reduce the longevity of the power source of the ICD.
Thus, in addition to providing an improved technique for determining an optimal ICD implantation configuration, it would be desirable to also provide an improved technique for determining the optimal DFT for programming an ICD which reduces the number of times that fibrillation needs to be induced in the patient, reduces the number and magnitude of electrical pulses administered to the heart of the patient during the determination process, and permits the DFT to be determined more precisely than many conventional techniques permit. It is to these ends that other aspects of the present invention are directed.
Hence, certain aspects of the invention are directed to determining an optimal ICD implantation configuration for a particular patient and then for determining the optimal DFT for programming the ICD. Other aspects of the invention are directed to determining the optimal DFT based upon a pre-determined ICD configuration. Such is particularly useful if an ICD has already been implanted in a patient. Still other aspects of the invention are directed to determining a suitable DFT for use with a patient based upon characteristics of the patient. Still other aspects of the invention are directed to determining whether a particular ICD configuration is acceptable for use with a patient given a predetermined DFT for the patient.
In accordance with a first aspect of the invention, a method is provided for determining the DFT for a patient. In accordance with the method, predictive information correlating patient clinical data with tissue defibrillation thresholds is maintained. Patient data for a particular patient is then received. The defibrillation threshold for the particular patient is automatically determined based upon the data input for the patient in combination with the predictive information correlating patient clinical data with tissue defibrillation thresholds.
In an exemplary embodiment, the predictive information correlating patient clinical data with DFT includes one or more of gender vs. threshold, ventricular fibrillation history vs. threshold, heart size vs. threshold, chest size vs. threshold, left ventricular mass vs. threshold, atrial mass vs. threshold, and medication usage vs. threshold, ejection fraction vs. threshold, NY classification vs. threshold. Patient data input for the patient includes corresponding information such as the gender of the patient, any history of ventricular fibrillation in the patient, the heart size, left ventricular mass, atrial mass, chest size of the patient, and any medications expected to be used by the patient.
Preferably, the patient clinical data is input into, and maintained within, an ICD programmer used in connection with programming the ICD. The patient clinical data is arranged in a set of tables which correlate DFT with various ranges of the aforementioned groups of patient clinical data, such as gender, chest size, atrial and ventricular mass, and the like. The parameters for the patient under consideration is then input by a physician or other medical personnel into the programmer which automatically applies the patient data to the predictive information stored within the tables and determines therefrom the expected DFT for the patient, which can then be displayed using a display screen of the programmer for use by the physician in determining an optimal implantation configuration for the ICD, including the setting of any programmable DFT parameters of the ICD.
In accordance with a second aspect to the invention, a method for determining the DFT for a particular patient based upon images of the thorax of the patient is provided. In accordance with the method, the thorax is imaged, using MRI or similar technique, then the thorax of the patient is modeled based upon the images of the patient to generate a patient model. Conduction characteristics of electrical pulses in the patient are determined based upon the patient model. Then, the DFT for the patient is determined based upon the conduction characteristics.
In an exemplary embodiment, the thorax of the patient is modeled, in part, by employing finite element analysis to extract the shape and boundary of various structures in the thorax, including the individual chambers of the heart and the major arteries and veins in the vicinity of the heart and possibly further including the lungs, the rib cage, the sternum, any fat disposed in the vicinity of the heart, thoracic muscles and cutaneous fat. Once the structure boundaries are determined, an electrical conductivity value is assigned to each structure boundary, then differential equations representative of electrical conduction characteristics are solved to estimate the electrical field strength, current density, or electrical potential within the heart of the patient. The DFT is then determined based upon the estimates of the electrical field strength, the current density, or the electrical potential. Thus, in the exemplary embodiment, the DFT for the patient is determined directly from images generated of the thorax of the patient using modeling techniques.
Thus, in accordance with either the first or second aspects of the invention, the DFT for a patient is determined without having to repeatedly trigger fibrillation within the heart of the patient, then administer shocks of differing dosages in an attempt to terminate fibrillation. If desired, techniques drawn from both the first and second aspects of the invention can be combined to yield an even more reliable determination of the DFT for the patient. Also, although the techniques of the first and second aspects of the invention eliminate the need to defibrillate a patient to determine the tissue defibrillation threshold, it may nevertheless be desirable to defibrillate the patient at least once using the threshold that has been determined, to verify that the threshold is correct. It may also be desirable to utilize a safety margin as well in connection with the DFT to further ensure reliable defibrillation. The safety margin will likely be less than that which would be employed in connection with a defibrillation threshold determined using conventional techniques.
In accordance with a third aspect of the invention, a method is provided for determining an implantation configuration capable of achieving a DFT for an ICD to be implanted within a patient. In accordance with the method, predictive information correlating ICD implantation configurations with tissue defibrillation thresholds for a population of patients is maintained. The DFT for a particular patient is then input or otherwise determined. Then, an implantation configuration capable of providing defibrillation pulses achieving the DFT is automatically determined based upon the threshold for the particular patient in combination with the predictive information correlating ICD implantation configurations with defibrillation thresholds.
In an exemplary embodiment, the predictive information correlating ICD implantation configuration with tissue defibrillation thresholds includes one or more of lead type, lead position, ICD position, lead polarity, auxiliary wire positioning, and the amount of energy the ICD can provide in a single defibrillation pulse and the extend to which the ICD is programmable.
Preferably, the programmer further identifies an optimal implantation configuration. The programmer may, for example, identify the configuration requiring the least amount of energy to be generated by the ICD to achieve the DFT. Alternatively, the programmer may identify the configuration providing the lowest overall costs while also being capable of achieving the DFT. As another example, the programmer may identify the configuration providing the greatest device longevity while also being capable of achieving the DFT. As can be appreciated, the optimal configuration can be defined in a number of ways and, preferably, the programmer displays information identifying various configurations satisfying different optimal criteria.
Thus, various techniques are provided for determining the DFT for a particular patient and for automatically determining an acceptable, or perhaps optimal, implantation configuration sufficient to meet the DFT to ensure that the threshold is reliably met in the event fibrillation occurs within the patient. Hence, fibrillation need not be repeatedly induced within the patient and the patient need not be repeatedly shocked to terminate fibrillation. Accordingly, costs, risks and dangers inherent with those procedures are substantially eliminated. Numerous other advantages and features of the invention are also described herein. Apparatus embodiments of the invention are also provided.